FIG. 1 shows an optical modulator based upon the Mach-Zehnder interferometer principle, commonly referred to as a MZI modulator. The modulator includes an optical waveguide receiving an optical signal with power Pin, which is divided into two branches 12a and 12b at a point S. The two branches come together again at a point J. Each branch carries half of the original optical power.
Each branch comprises a static electro-optical phase shifter (SPS), i.e. (SPSa and SPSb), and a dynamic electro-optical phase shifter (DPS), i.e. (DPSa and DPSb). The static phase shifters SPS are used to define an initial phase difference φ0 between the two optical waveguide branches. They are controlled by respective bias signals IBa and IBb. The dynamic phase shifters DPS are used to perform a differential modulation around the initial conditions defined by the SPS phase shifters. They are controlled by respective modulation signals M and M/ varying in phase opposition. The waves from both branches of the modulator are added at point J. The resulting wave has a power of Pin·cos2(Δφ/2), neglecting the optical losses, where Δφ is the instantaneous phase difference between the waves of the two branches.
FIG. 2 shows the waveguide branches 12a and 12b incorporating phase shifters SPS and DPS, shown in gray. As shown, the waveguides are formed in transparent islands, made of intrinsic semiconductor material, having an inverted “T” section, the central portion of which transmits the optical beam. The phase shifters are configured to replace waveguide segments and have the same inverted “T” cross-section. The edges of the phase shifters bear electrical contacts used to control the phase shifters—they usually extend above the plane of the waveguide, as shown, to reach the metal levels.
FIG. 3A shows a DPS phase shifter referred to as a high-speed phase modulator (HSPM). The cross-section plane is perpendicular to the axis of the optical waveguide. A circle indicated with a dashed line, at the thicker central region, represents the area crossed by the optical beam. The phase shifter comprises a semiconductor structure, typically silicon, forming a P-N junction 14 in a plane parallel to the axis of the waveguide, and offset relative thereto. The junction 14 is shown, for example, at the right side face of the waveguide. A P-doped zone extends to the left of junction 14, which has a cross-section conforming to the cross-section of the waveguide, namely elevated in the center and lower at the edge. Zone P ends at its left by a P+ doped raised area, bearing an anode contact A. An N-doped zone extends to the right of the junction 14, conforming to the cross section of the waveguide. The zone N ends to the right by an N+ doped raised area, bearing a cathode contact C. The structure of the phase shifter may be formed on an insulating substrate, for example, a buried oxide layer BOX.
For controlling the phase shifter of FIG. 3A, a voltage is applied between the anode and cathode contacts A, C, which reverse biases the junction 14 (the ‘+’ on the cathode and the ‘−’ on the anode). This configuration causes a displacement of electrons e from the N region to the cathode and of holes h from the P region to the anode, and the creation of a depletion region D in the vicinity of the junction 14. The carrier concentration is thus based upon the magnitude of the bias voltage in the area crossed by the optical beam, which results in a corresponding modification of the refractive index of this area.
FIG. 3B shows a P-I-N junction SPS phase shifter. The P and N-doped central regions of the structure of FIG. 3A have been replaced by a single intrinsic semiconductor zone I, in practice, a zone having a minimal P doping level. For controlling this phase shifter, a current is applied between the anode and cathode contacts A and C, which forward biases the junction (the ‘−’ on the cathode and the ‘+’ on the anode). A current is established between the anode and the cathode, thereby causing the injection of carriers in the intrinsic zone I (holes h from the P+ region to zone I and electrons e from the N+ region to zone I). The carrier concentration, i.e. the refractive index, is thus changed as a function of the current in the area crossed by the optical beam. PIN phase shifters have a slow response compared to HSPM shifters, but they offer a wider range of adjustment, which is why they are used to set the quiescent conditions of the modulator.
An MZI modulator may achieve in theory a modulation frequency of several tens of gigahertz. However, this frequency may be affected by the matching between shifters DPSa and DPSb, and the matching between the control signals M and M/. As shown in FIG. 2, the shifters DPSa and DPSb may be inserted into separate waveguide branches. The fabrication constraints for these branches may impose a gap between the branches of the order of a millimeter, which is a considerable distance at the scale of the integration technologies used for the circuits producing the control signals. As a result, it may be difficult to achieve the desired optical and electrical matching.